The invention relates to an implantable electrode for stimulation of nerves adapted to be surgically implanted around a nerve bundle.
Restoring motor-functions of (partially) paralysed people is known by exploiting the fact that muscles respond to electrical energy. This principle can be utilised to directly stimulate muscles or to stimulate the nerves leading to the muscles. Direct stimulation of muscles requires a number of electrodes distributed on the muscle, compare e.g. U.S. Pat. No. 5,314,458, and is relatively power consuming. In addition, the control equipment is rather complicated for surveillance of the electrodes to achieve the desired movement of the muscle.
Stimulation of nerves can be made by placing electrodes locally around the nerves which is known from e.g. U.S. Pat. No. 5,038,781. For this reason the surgical intervention is minor. Stimulation of nerves also requires much less energy than stimulation of muscles although the power requirement is still high compared to e.g. a pacemaker due to the higher stimulation frequency necessary to obtain smooth contractions of muscles. The present invention relates to nerve stimulation.
The controlling equipment and the power supply are placed externally on the human body and could be connected to the electrodes by using wires passing through the skin which for obvious reasons is not an attractive solution. Alternatively, the placement of a transmitter on the body and an implant of a receiver in the body wired to the electrodes is also known, said transmitter sending signals and energy through the skin and flesh to the receiver. More specifically, the present invention relates to the latter type of stimulator electrodes.
Such a neurological stimulation apparatus is known from e.g. U.S. Pat. No. 5,038,781 said apparatus comprising nerve cuffs with electrodes attached to a nerve bundle and wire connected to an implant case containing the electronic circuitry receiving operating power and information signals by radio frequency coupling to an external unit. Running wires under the skin and implanting a casing in the body is for obvious reasons not an attractive solution.
Another example is known from U.S. Pat. No. 4,057,069 which deals with an implantable nerve stimulator receiving energy from an external unit by telemetrically transferring energy from an external unit. The nerve stimulator contains a detector circuit which by an adequate excitation can address a number of electrode pairs of a multiplexer. The nerve stimulator has among other things the disadvantage in connection with multielectrode systems of not being able to address all of these simultaneously, however, only sequentially. In addition, the nerve stimulator is limited locally, as the electrode pairs in question necessarily have to be in galvanic contact with the nerve stimulator itself.
A further example is known from U.S. Pat. No. 3,667,477 which deals with an implantable nerve stimulator which receives energy from an external unit by telemetric transmission. The implantation of the disclosed nerve stimulator, however, is quite complicated, as the stimulator is made of separate receiving and electrode means coupled by leads.
It could be mentioned that it is also known to stimulate muscles by means of telemetric transmission.
An example is known from U.S. Pat. No. 5,314,458 which deals with an implantable muscle stimulator which receives energy from an external unit by telemetric transmission. This unit can be addressed individually, however, due to its placement on the muscle itself a very powerful excitation is needed and subsequently a very powerful electromagnetic field which causes the electromagnetic transfer of energy to be less attractive considering the continuing influence of the surrounding tissue. In addition, it is needed that the stimulator is being widely controlled when stimulating a muscle complex, as a certain number of specific controlled stimulators is needed in order to obtain an appropriate movement of the muscle.
One object of the present invention is to provide an improved and simplified neurological stimulation apparatus. A further object is to minimise the surgical intervention when applying the apparatus in the body.
When according to the present invention the input terminals coupled to one or more receiving coils are mounted on the stimulator housing, a remote individual addressable stimulator for stimulation of nerve bundles is obtained thus being able to function as an isolated implanted unit. When implanted, the stimulator will be placed in a juxta position or around the nerve being stimulated. According to the invention there is no need for any lead connections between the stimulator and external circuits, in the sense that the transmitter can be activated on location by remote signals without any need for an internal power source.
The control of the stimulator or the stimulators and the resulting activation of muscles is made even more simple due to the fact that a stimulation of the nerve often is less complicated than a stimulation directly on the muscles which are to be activated. In the case when a group of muscles or a muscle with multiple motor points innervated by the same nerve are wished to be activated at the same time (as for example in the case of dropfoot stimulation), it is advantageous to use one multichannel nerve stimulator placed on the nerve compared to many individual stimulators placed in the muscles.
According to the invention it is thus possible to obtain a remote stimulation using a minimal electromagnetic field.
Coupling the input terminals to one or more receiving coils mounted on the stimulator/electrode housing, it is possible to obtain a necessary power supply to the electrodes and the electronic circuit means which is provided telemetrically via the receiving coil or receiving coils, thus avoiding the use of lead connections between the internal part of the stimulator and the external part of the stimulator. It is thus possible to avoid battery powered electrodes.
Furthermore, the transmitter is basically free of maintenance in the sense that replacement of implanted power sources or the like is not needed.
In this connection, it is noted that the necessary effect needed to stimulate the nerves is approximately ten times lower than the necessary effect needed for direct stimulation of muscles. By stimulating the nerves rather than the muscles, a remote activation of the stimulator is particularly attractive.
The risk of complications due to the surgery activity required to implant the stimulator is moreover reduced in the sense that the stimulator itself, according to the invention, can be realised in a very compact design in contrary to e.g. lead-connected systems.
The stimulator according to the invention is especially applicable in situations where a continuing or periodically stimulation of the muscle is needed, as the implanted stimulator under constant influence of movement is mechanically as well as electronically stable and sturdy in the sense that neither the electrical nor the mechanical parts undergo stress during said movement which also results in a minimised risk of IN VIVO complications.
In addition, the use of an external power source will in practise result in a minimised risk of noise signals activating the stimulator as these external noise signals necessarily have to reach a certain noise level at a given wavelength in order to activate the electrodes of the stimulator.
Accordingly, in a further embodiment of the invention, the electrode means, when implanted around said nerve bundle, comprises one or more circle-shaped or semi-circumferential electrodes defining a contact area, each contact area being electrically coupled to the output terminals of the electronic circuit means. (Totally or partly surrounding the output terminals). This particular xe2x80x9cshapexe2x80x9d of the electrode means thus makes it possible to obtain a very well-defined positioning of the stimulator as regards the nerve bundle and at the same time, the electrode means create good contact between the electrode means and the nerve bundle.
When the electrode means, when implanted around said nerve bundle, comprises one or more ring-shaped electrodes totally or partly surrounding the nerve bundle, each ring-shaped electrode comprising at least two distinct electrode contact areas, each electrode area being electrically coupled to the output terminals of the electronic circuit means, it is possible to obtain a stimulator which is able to make a selective stimulation of the nerve and the associated muscles. The stimulation of the nerves can be obtained in various ways depending on which result is wanted. The stimulation can thus be obtained by a mono- or multipolar electrical stimulation, the chosen multipolar electrode stimulation being transverse, e.g. effected by electrodes on the same ring electrode or longitudinal, e.g. effected by electrodes on different rings. Furthermore, it is possible to combine transverse and longitudinal stimulation. The stimulator thus has the possibility of being used with an overall degree of selectivity which can be chosen from known mono- or multipolar configurations.
In a multichannel stimulator, each channel has its own electronic circuit connected to a separate receiver coil. The only difference between each channel is the resonance frequency of which each channel is active. The main purpose of the electronic circuit is to create an adequate stimulation pulse shape.
When each transmitter comprises one or more transmitting coils positioned in spaced relationships on each side of one or more of the stimulators, it is possible to obtain a very useful remote activated stimulator system.